1. Field of the invention
This invention relates to a biological signal measuring device which reproduces a biological signal waveform such as an electrocardiogram with high fidelity which is detected from an object under examination.
2. Related Art
One example of a conventional biological signal measuring device provided for an electrocardiogram is designed as shown in FIG. 5. In the device, signals detected through electrodes 1, 2 and 3 are amplified by an amplifier 4 and applied to a filter 5. The output signals of the filter 5, namely, electrocardiogram waveforms are applied to an A/D (analog-to-digital) converter 6, where they are converted into digital signals, which are applied to an output terminal 10. The waveform data provided at the output terminal 10 are processed, for instance, by a waveform displaying section, so that they are displayed on the screen of a CRT (cathode ray tube).
In order to obtain stable electrocardiograms, the filter 5 of the biological signal measuring device is made small in time constant.
As was described above, in the conventional biological signal measuring device, the biological signals are applied to the filter 5 short in time constant to obtain stable waveforms. Hence, when a triangular pulse wave as shown in FIG. 3 (a) or in FIG. 4 (a) is applied to the filter 5, the output waveform of the filter 5 is distorted as shown in FIG. 3 (b) or in FIG. 4 (b).
Therefore, when it is required to observe the ST segment of an electrocardiogram for the diagnosis of the ischemia due to cardiac infarction or angina pectoris, the following difficulty is involved: the ST segment are distorted, and accordingly it is difficult to analyze the waveforms.
Furthermore, for instance in the case of measuring an cardiac output, a preparatory operation must be carried out; that is, before the measurement is started, a zero point adjustment must be carried out to prevent the distortion of the waveform. The preparatory operation to be carried out before the measurement is rather trouble.